Several publications and patent documents are referenced in this application in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications and documents is incorporated by reference herein.
Non-radioactive detection of nucleic acids utilizing fluorescent labels is an important technology in molecular biology. Many procedures employed in recombinant DNA technology previously relied heavily on the use of nucleotides or polynucleotides radioactively labelled with, for example 32P. Radioactive compounds permit sensitive detection of nucleic acids and other molecules of interest. However, there are serious limitations in the use of radioactive isotopes such as their expense, limited shelf life and more importantly safety considerations. Eliminating the need for radioactive labels enhances safety whilst reducing the environmental impact and costs associated with, for example, reagent disposal. Methods amenable to non-radioactive fluorescent detection include by way of non-limiting example, automated DNA sequencing, hybridization methods, real-time detection of polymerase-chain-reaction products and immunoassays.
For many applications it is desirable to employ multiple spectrally distinguishable fluorescent labels in order to achieve independent detection of a plurality of spatially overlapping analytes. In such multiplex methods the number of reaction vessels may be reduced simplifying experimental protocols and facilitating the production of application-specific reagent kits. In multi-colour automated DNA sequencing for example, multiplex fluorescent detection allows for the analysis of multiple nucleotide bases in a single electrophoresis lane thereby increasing throughput over single-colour methods and reducing uncertainties associated with inter-lane electrophoretic mobility variations.
However, multiplex fluorescent detection can be problematic and there are a number of important factors which constrain selection of fluorescent labels. First, it is difficult to find dye compounds whose emission spectra are suitably spectrally resolved. In addition when several fluorescent dyes are used together, simultaneous excitation may be difficult because the absorption bands of the dyes are usually widely separated. Many excitation methods use high power lasers and therefore the dye must have sufficient photo-stability to withstand such laser excitation. A final consideration of particular importance in molecular biology methods is that the fluorescent dyes must be compatible with the reagent chemistries used such as for example DNA synthesis solvents and reagents, buffers, polymerase enzymes and ligase enzymes.
As sequencing technology advances a need has developed for further fluorescent dye compounds, their nucleic acid conjugates and dye sets which satisfy all of the above constraints and which are amenable particularly to high throughput molecular methods such as solid phase sequencing and the like.
Application WO2007135368 describes a class of rhodamine compounds suitable for use as fluorescent labels. The compounds described therein are suitable for use in solid phase nucleic acid sequencing protocols. Advances in the technology and throughput of solid phase nucleic acid sequencing have led to further developments and improvements to the molecular design of fluorescent labels, particularly in the context of the interaction between the fluorescent reagents and particular nucleic acid sequences.
Fluorescent dye molecules with improved fluorescence properties (such as fluorescence intensity, position of fluorescence maximum and shape of fluorescence band) can improve the speed and accuracy of nucleic acid sequencing. Fluorescence signal intensity is especially important when measurements are made in water based biological buffers and/or at higher temperature as fluorescence of most dyes is significantly lower at such conditions. Moreover, the nature of the base to which a dye is attached also affects the fluorescence maximum, fluorescence intensity and other spectral dye properties. The sequence specific interactions between the fluorescent dye and the nucleobase can be tailored by specific design of the fluorescent dyes. Optimisation of the structure of the fluorescent dyes can improve their fluorescent properties and also improve the efficiency of nucleotide incorporation, reduce the level of sequencing errors and decrease the usage of reagents in, and therefore the costs of, nucleic acid sequencing.
Described herein are improved rhodamine constructs and their use as bio-molecule labels, particularly as labels for nucleotides used in nucleic acid sequencing. The improvements can be seen in the higher fluorescence intensities of such dyes when prepared as bio-molecule conjugates and in the length and quality of sequencing read obtainable using the new fluorescent constructs.